1. Filed of the Invention
The present invention relates to a pump impeller manufactured by press forming of steel plate, which is advantageously used as an impeller of a centrifugal pump made of steel plate.
2. Prior Art
Conventional pump impellers manufactured by press forming of steel plate are generally classified into two groups with respect their structure, namely, (a) two engaging planes type, and (b) boss welding type.
FIGS. 6(a) and 6(b) are a front view and a side sectional view, respectively, of a main portion of an impeller according to the two engaging planes type (a). In this type, a retainer plate 2 is fixed to the central portion of the impeller main shroud 1. The main shroud 1 and the retainer plate 2 are both formed with circular bores 3 having two parallel engaging planes for fitting to a main shaft (not shown) having the same cross-sectional shape, and connected with each other in an axial direction by means of screws. In the figures, numeral 4 denotes a side plate, and numeral 5 an impeller blade.
FIGS. 7(a) and 7(b) are a front view and a side sectional view, respectively, of a main portion of an impeller according to the boss welding type (b). In these figures, the same numerals as in FIGS. 6(a) and 6(b) indicate the same or similar portions. In this type, a boss 6 is formed, through such as machining, with a slunt or conical surface 6a on the front side (suction side) of the impeller, a step portion 6b on the rear end of the conical surface 6a, and a cylindrical surface 6c of a smaller diameter adjacent to the step portion 6b. The impeller main shroud 1A is formed with a central aperture. The main shroud is fitted to the boss with its central aperture fitting on the cylindrical surface 6c and with its central portion abutting on the step portion 6b, and then the main shroud and the boss are connected with each other by welding 7 between the rear portion of the conical surface 6a and the abutting portion of the main shroud 1A.
In the impeller of the two engaging planes type of the prior art shown in FIGS. 6(a) and 6(b), however, the impeller main shroud 1 and the main shaft are engaged with each other through the two circular holes 3 having two engaging planes. In this case, therefore, a torque for the impeller is borne only by a small area extending over the thickness of the main shroud 1 and the retainer plate 2. In consequence, the impeller of this type can not be used, when the required power or torque is great, and further the impeller of this type is easily affected by vibration due to such as a radial thrust or force. In addition, the shaft is required to have a special cross-sectional shape and, thus needs troublesome machining.
In the impeller of the boss welding type of the prior art shown in FIGS. 7(a) and 7(b), the impeller mainplate 1A and the boss 6 are fitted to each other and welded to each other. In this case, however, since arc welding is required around the periphery of the boss for assuring a sufficient strength, there is great thermal deformation caused, and therefore, the central hole of the boss is required to be remachined after welding. On the other hand, if the boss and the main shroud are welded to each other by projection welding at several positions as shown in FIG. 7(a), the remachining may not be required, but the impeller cannot withstand great torque or power because of an insufficient strength at the welded points.
Further, in both of the above-mentioned welding cases, there is the problem that high reliability can not be expected, because the welding portion is positioned at the regions where stress concentrations occur.